Conventional photodiodes as employed in complimentary metal-oxide semi-conductor (CMOS) image sensors suffer from a set of practical limitations such as elevated dark current densities that can be overcome by the use of a pinned (or buried photodiode (PPD). Unfortunately, the advantage of the PPD regarding low dark current is obtained at the expense of a limited dynamic range of typically less than 80 dB. In conventional photodiode pixels, these limitations of the dynamic range can be overcome with the use of a sub-linear device connected the cathode of the photodiode, as taught for example in U.S. Pat. No. 6,815,685 (M. Wäny et al., “Photodetector and method for detecting radiation”). However this solution cannot be adopted for PPDs and charge-coupled devices (CCDs) because the storage node (ST) is internal to the device. By setting the voltage of the transfer gate (TG) to a well defined voltage to allow flowing excess charge into the floating diffusion (FD), a sub-linear element connected to FD generates a voltage on said node that is a sub-linear measure of the photo current.
International patent application WO2007/115415 (Seitz et. al, “Photo sensor with pinned photodiode and sub-linear response”) discloses a photo sensor exhibiting low noise, low smear, low dark current and high dynamic range that consists of a pinned (or buried) photodiode (PPD) with associated transfer gate (TG), a reset circuit and a device (SL) with sub-linear voltage-to-current characteristic. The storage node (Se) is external to the device.
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